Among internal combustion engines, there are some which include an apparatus for controlling the number of idle rotations. The apparatus for controlling the number of idle rotations includes a bypass air inlet passage for providing communication between points in an air inlet passage which are on opposite sides of an air inlet throttle valve of the internal combustion engine so as to bypass the throttle valve, and a control valve for controlling the quantity of air passing through the bypass passage, the bypass air inlet quantity control valve being actuated by a duty ratio signal in order to control the number of idle rotations through feed-back so that the number of engine revolutions becomes a target number of idle rotations. The apparatus for controlling the number of idle rotations of the internal combustion engine, as shown in FIGS. 5 and 6, controls the number of idle rotations to a target number less than the number of rotations for starting the controlling of the idle rotations so that the number of engine revolutions becomes the target number of idle rotations, which target number is set in accord with the temperature of the cooling water. Also, the apparatus for controlling the number of idle rotations, as shown in FIGS. 7 and 8, starts the controlling of the number of idle rotations so that the number of engine revolutions becomes the target number of idle rotations when each and every one of certain conditions for starting the controlling of the number of idle rotations is satisfied, the conditions relating to the opening degree of an air inlet throttle valve of the internal combustion engine, the vehicle speed, and the number of engine revolutions.
More specifically, the apparatus for controlling the number of idle rotations starts controlling the number of idle rotations so that the number of engine rotations becomes the target number of idle rotations if each and every one of the following conditions is satisfied: the idle switch has been turned on to indicate that the opening degree of the air inlet throttle valve is the idle opening degree and a delay time has passed (idle switch ON+delay time); the vehicle speed detected by a vehicle speed sensor is less than the vehicle speed for starting the controlling of the number of idle rotations (vehicle speed&lt;vehicle speed for staring the idle controlling); and the number of engine revolutions detected by an engine speed sensor is less than the number of rotations for starting the idle controlling (number of engine revolutions&lt;number of rotations for starting the idle controlling). In addition, the apparatus for controlling the number of idle rotations also starts the controlling of the number of idle rotations so that the number of engine rotations becomes the target number of idle rotations if each and every one of the following conditions is satisfied: the idle switch is turned on to indicate that the opening degree of the air inlet throttle valve is the idle opening degree (idle switch ON); the number of engine revolutions is less than the target number of idle rotations (number of engine revolutions&lt;target number of idle rotations); the vehicle speed detected by a vehicle speed sensor is less than the vehicle speed for starting the controlling of the number of idle rotations (vehicle speed&lt;vehicle speed for starting the idle controlling); and the number of engine revolutions detected by an engine speed sensor is less than the number of rotations for starting the idle controlling (number of engine revolutions&lt;number of rotations for starting the idle controlling).
Also, when the controlling of the number of idle rotations is subsequently stopped because one of the conditions for starting the controlling of the number of idle rotations is no longer satisfied, for example where the vehicle speed becomes equal to or greater than the vehicle speed for starting the idle controlling, the duty ratio of the signal controlling the bypass air inlet quantity control valve is maintained at a fixed value. There are two approaches for maintaining the duty ratio at a fixed value. In one case, the duty ratio is maintained at a preset fixed value, whereas in the other case a duty ratio being used to control the number of idle revolutions just before the controlling of the number of idle rotations is stopped is memorized and the duty ratio is thereafter maintained at the memorized value.
As such an apparatus for controlling the number of idle rotations, there is that disclosed in Japanese Patent Early Laid-Open Publication No. Sho 63-215855. The apparatus disclosed in this Publication is intended to prevent an increase in the number of idle rotations under a no load driving condition when it is shifted from a state where the bypass air inlet quantity is large to a state where the quantity is small, and to rapidly stabilize it at a predetermined number of idle rotations.
In a case where the opening degree of the air inlet throttle valve becomes the idle opening degree as a result of turning on of the idle switch, as in a case where the vehicle is running at a reduced speed and the clutch is separated to bring the clutch switch to its off position so that the vehicle speed detected by the vehicle speed sensor is at or above the speed level below which starting of the idle controlling occurs, the variable controlling of the bypass air inlet quantity control valve, as mentioned above, is stopped and the duty ratio of the signal controlling the bypass air inlet quantity control valve is maintained at a fixed value and the number of engine revolutions (engine speed) is decreased toward the target number of idle rotations.
At this time, however, owing to the application to the engine of loads such as electric loads and air conditioning equipment as an auxiliary device, if the quantity of bypass air passing through the bypass air inlet quantity control valve (determined by the duty ratio set to the fixed value) is less than the required bypass air inlet value, the number of engine rotations is excessively decreased to a value below the target number of idle rotations, thus sometimes resulting in engine stall. Also, if the bypass air quantity passing through the bypass air inlet quantity control valve (determined by the duty ratio set to the fixed value) is larger than the required bypass air inlet quantity, the number of engine revolutions does not decrease all the way to the target number of idle rotations, and to the contrary may sometimes increase.
In this case, there may be additionally provided a load switch for detecting the application of electric loads and the like, so that the controlling of the number of idle rotations is started when an additional condition is satisfied, namely that the load becomes greater than a load above which starting of idle controlling is permitted. However, there is a disadvantage in this arrangement, in that a cost increase is involved owing to the additional employment of a load switch.
Therefore, an object of the present invention is to provide an apparatus for controlling the number of idle rotations of an internal combustion engine which is capable of preventing an excessive decrease of the number of engine rotations, engine stall and the like occurring when the controlling of the number of rotations is started after controlling of the number of idle rotations is stopped, and in which a cost increase due to the additional provision of a load switch is avoided.